MgO is used for various purposes including heat-resistant applications (see the following Patent Documents 1 and 2) because it is a material having excellent heat conductivity, heat resistance, chemical stability, oxidation resistance and isolation performance
MgO is relatively excellent in sinterability, and capable of obtaining denseness of up to almost 99% in terms of relative density, even by normal sintering. In reality, it is difficult to increase a sintered density of an MgO sintered body to a theoretical density thereof, and pores, such as micropores and several-micrometer pores, remain in the sintered body. It is contemplated to increase a sintering temperature with a view to improvement in sintered density (reduction in pores). However, if the sintering temperature is increased while placing a top priority on the improvement in sintered density, crystal grain growth will be promoted, resulting in a situation where pores remain in coarse crystal grains. Such pores are hardly eliminated even by a subsequent hot isostatic pressing (HIP) treatment at high temperatures and pressures.
As above, the conventional MgO sintered body is insufficient in sintered density thereof, and grain growth will occur if it is attempted to increase the sintered density. Thus, when it is used, particularly, as a jig or a structural member such as a heat-insulating board, there are the following problems.
1) Deterioration in Mechanical Properties
(1) Deterioration in Strength
Strength includes bending strength, compressive strength and shearing strength, each of which depends on remaining pores inside the sintered body. Further, coarse grains resulting from grain growth during sintering are also likely to become an origin of destruction or breakage. The insufficient strength due to the pores and the grain growth gives rise to fatal damage, such as fracture or chipping, in use as a structural member.
(2) Deterioration in Hardness
The presence of the pores and the grain growth also poses a risk of deteriorating hardness, and thereby leads to deterioration in abrasion resistance, which causes a reduction in usable life of the structural member due to abrasion.
2) Deterioration in Surface Smoothness
The presence of the pores and the grain growth inside the sintered body means that a surface smoothness is deteriorated. In use as a structural member, there are many applications requiring a high surface smoothness in a working surface. If the surface smoothness is low, the structural member has problems, such as (1) a problem that pores in a sliding surface triggers chipping to encourage deterioration in the surface smoothness, resulting in a reduction in usable life of the structural member; and (2) a problem that the deterioration in the surface smoothness causes an increase in friction coefficient, resulting in the occurrence of a trouble, such as abnormal heat generation, or reaction or adhesion with a counterpart member.
3) Deterioration in Heat Conductivity
MgO has characteristics of high heat conductivity. One factor spoiling the heat conductivity is the presence of pores. Specifically, if there are pores or impurities in grain boundaries, heat conduction is hindered to preclude a possibility to obtain the inherent heat conductivity. Thus, as a prerequisite to obtaining high heat conductivity, it is necessary to reduce the pores, i.e., increase a relative density of the sintered body to almost 100% of the theoretical density.
4) Contamination of Atmosphere Due to Gas Generation
Gas of a sintering atmosphere is trapped in each pore residing in the sintered body. For example, in sintering under ambient atmosphere, components of ambient air, such as nitrogen gas, carbon dioxide and oxygen, will be trapped to form pores. In sintering under an argon or nitrogen gas atmosphere, the gas will be trapped to form pores. If grain boundaries of the sintered body are softened during use in a high-temperature range, the trapped gas will be released from the sintered body. Particularly in applications refusing even a slight amount of imparities, such as semiconductor manufacturing, the released gas causes a critical defect.
The MgO sintered body is also frequently used as a sputtering target (see the following Patent Documents 3 and 4). In this sputtering target application, the improvement in the mechanical properties and the heat conductivity is a key factor in preventing breaking and peeling during sputtering, and the reduction in gas generation from the sintered body is a key factor in preventing contamination of an atmosphere inside a sputtering apparatus.
[Patent Document 1] JP 7-133149A
[Patent Document 2] JP 2006-169036A
[Patent Document 3] JP 10-158826A
[Patent Document 4] JP 2005-330574A